go: mux: log more about the packet on error
go: mux: log more about the packet on error

If we receive a packet with a bad channel id, this will print the
type of packet it was.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit